The present invention relates to techniques for controlling a tape transport and more specifically, with enhanced regulation of tape tension while suppressing time-varying or periodic tension disturbances.
Tape systems can provide cost effective storage solutions, which are becoming increasingly important as the world faces an explosive growth in the rate of data creation. In recent years, the capacity and performance of tape storage systems have increased considerably, and the potential for further growth appears to be substantial. To achieve higher tape cartridge capacities and improved performance, advances in several technical areas are necessary. For example, areal density increase, i.e., increase in linear and/or track density, is an essential requirement for achieving higher capacities. However, higher track density implies narrower track width, narrower write/read head elements, and closer tape-to-head spacing, leading to losses in signal-to-noise ratio and stringent requirements on the precision of the write/read head elements.
Deviations of tape tension and velocity from the target values may adversely affect the position error signal and hence the performance of a track following servo. Robust tape transport and track-following servomechanisms are therefore necessary to provide good read-channel performance on all parallel data channels during tape operation. Moreover, moving to thinner tape media, which leads to higher volumetric density, represents a further avenue for achieving higher capacities of tape cartridges. Hence, a need arises for the tape transport servomechanism to provide tighter control of tension, to prevent tension variations that might overstress the thin media.
A further problem may arise while operating in the steady-state velocity mode, in that periodic variations of tape tension around the nominal value, also called wrap-arounds, may occur. Such variations may be induced by the reel eccentricities. In tape transport, this problem may be particularly serious when the reel rotation frequencies approach the resonance frequency determined by the tape path. Although the adverse effects of tension variations are well acknowledged and understood, no effective conventional method has been developed for the suppression of these periodic tension disturbances with slowly time-varying frequency. Recent work on tape transport control with feedback of velocity and tension has focused on the choice of time-varying p-type controllers to achieve a closed-loop system behavior that is essentially independent of the tape longitudinal position. However, a p-type controller does not provide enhanced rejection of disturbances due to periodic tension variations.
Accordingly, a need arises for techniques by which a tape transport control system may provide enhanced regulation of tape tension while suppressing time-varying or periodic tension disturbances.